Integrators need a way to request Google as a sign in option

[mkhandekar]

Some integrators, such as Open Opportunties and Communicart, need their visitors to be able to sign in with Google. These integrators will have to explicitly request Google as a sign in option. Then Google will appear as the default sign in method.

See #738 (Sign in flow — sign in from Integrator website using Google) for the full flow, and key screens are below.

2.3 Connect With Google (by default)

A [Connect with Google] button and a text link that allows visitors to [sign in with any email] that takes them to the following screen.

2.2 Sign in with email (secondary choice)

A place for visitors to sign in with any email and a text link that allows them to [Connect with Google] that takes them to the previous screen.

We need:

• a spot in the database to store whether Google should be enabled as a login
• a test for that field
• the Google log in screen to show up as the default (and email log in to be secondary) when this option is chosen by an integrator

Part of #735: Sign in flow — to code
cc: @yozlet @harrisj @juliaelman